1. Field of the Invention
The present invention relates generally to an order wire, which is an auxiliary wire for use in the installation and maintenance of communication equipment, and more particularly to a simple method of producing a ring-back tone in an order wire. The term "ring-back tone" herein means a signal tone informing the transmitter side of the fact that a calling signal is being sent to the receiver side.
2. Description of the Related Art
A dual-tone multifrequency signal (DTMF signal) is known as a signal used in transmitting a calling signal and a ring-back tone signal over a communication line (including the order wire) such as the telephone line. The DTMF signal is that which uses a combination of one frequency out of low-group frequencies and one frequency out of high-group frequencies for representing a numeral or a symbol, and the signal is identified and the numeral or symbol is regenerated by a circuit formed of a suitable filter and the like.
In a communication system including a plurality of stations, there is sometimes established an order wire for installing and maintaining communication equipment between any two stations (the calling station and the called station). In that case, the calling station calls the called station with a DTMF signal and the called station notifies the calling station of the fact that it is being called using a voice calling function of the order wire. In ordinary communication lines, the called station sends a ring-back tone to inform the calling station of its being called. However, since it is normal practice not to use an exchange apparatus in an ordinary order wire, when a called station sends out a ring-back tone signal, the ring-back tone signal is received not only by the calling station but also by a plurality of other stations and trouble is caused. In order that the ring-back tone is produced only in the calling station, exchange techniques are required but, then, the cost for constructing the system increases.